The present invention relates to polysiloxane-containing polyurethane elastomeric compositions having improved properties which make them useful for a variety of applications, in particular the manufacture of medical devices, articles or implants which contact living tissues or bodily fluids.
Polyurethane elastomers represent an important class of segmented copolymers with excellent mechanical properties including high tensile strength, good tear and abrasion resistance and a relatively good stability in biological environments. Accordingly, polyurethanes are widely used in medical implants such as cardiac pacemakers, catheters, implantable prostheses, cardiac assist devices, heart valves and vascular grafts. The excellent mechanical properties of segmented polyurethanes are attributed to their two phase morphology derived from microphase separation of soft and hard segments. In polyurethanes used for long term medical implants, the soft segments are typically formed from a polyether macrodiol such as polytetramethylene oxide (PTMO) whereas the hard segments are derived from a diisocyanate such as 4,4xe2x80x2-methylenediphenyl diisocyanate (MDI) and a diol chain extender such as 1,4-butanediol.
Although PTMO-based polyurethanes are the materials of choice for a wide variety of medical implants, in some cases the polyurethanes degrade causing malfunction or failure of the implant. Degradation is usually apparent in terms of surface or deep cracking, stiffening, erosion or the deterioration of mechanical properties such as flexural strength1. The mechanisms responsible for such degradations include environmental stress cracking, the generation of cracks and crazes produced by the medium acting on the polyurethane at certain stress levels and metal ion induced oxidation. It is generally accepted that ether linkages in the PTMO-based elastomers are the most vulnerable sites for initiation of degradations2. Efforts have been made to overcome this problem by developing polyurethanes that are not exclusively based on PTMO such as those disclosed in Australian Patent No. 657267, U.S. Pat. No. 4,875,308 (Courey et al), U.S. Pat. No. 5,133,742 (Pinchuck) and U.S. Pat. No. 5,109,077 (Wick). Nevertheless, the combination of degradation resistance, mechanical characteristics, processability and clarity is suboptimal for certain applications. For example, there has been a long felt need for polyurethanes combining low durometer hardness, high flexibility, good processability and high resistance to degradation within the pacing industry for the insulation of leads. With the polyurethanes described in the aforementioned patents, there appears to be a lower limit to flexibility and Shore hardness, below which degradation resistance and/or mechanical properties are adversely affected.
Polycarbonate macrodiols have also been used as reactive ingredients in the synthesis of block and segmented copolymer systems, in particular high performance polyurethanes. Processes for preparing polycarbonate macrodiols based on a range of bishydroxy alkylene compounds are disclosed in JP 62,241,920 (Toa Gosei Chemical Industry Co. Ltd.), JP 64,01,726 (Dainippon Ink and Chemicals, Inc.), JP 62,187,725 (Daicel Chemical Industries, Ltd.), DE 3,717,060 (Bayer A. G.), U.S. Pat. No. 4,105,641 (Bayer Aktiengesellschaft), U.S. Pat. No. 4,131,731 (Beatrice Foods Company) and U.S. Pat. No. 5,171,830 (Arco Chemical Technology).
Polysiloxane-based materials, especially polydimethyl siloxane (PDMS) exhibit characteristics such as low glass transition temperatures, good thermal, oxidative and hydrolytic stabilities, low surface energy, good haemocompatibility and low toxicity. They also display an improved ability to be bonded to silicone components, by such procedures as gluing, solvent welding, coextrusion or comolding. For these reasons PDMS has been used in biomedical applications3,4. However, PDMS-based polymers generally have limitations and do not exhibit the necessary combination of tear resistance, abrasion resistance and tensile properties for many types of implants intended for long term use. It would be desirable for polymers to be available with the stability and biological properties of PDMS, but the strength, abrasion resistance, processability and other physical properties of polyurethanes. Polyurethanes incorporating PDMS would appear to fulfil this need, but to date, despite much experimentation, no compositions have been produced with the optimal combination of physical and biological properties.
Previous attempts to incorporate PDMS into polyurethanes have not been very successful5. Speckhard et al6 have indicated that as a result of the large differences in solubility parameters between polysiloxane and hard segments, PDMS-based polyurethanes are likely to be highly phase separated materials having poor mechanical properties. As a consequence of a large difference in polarity between hard and soft segments, it is anticipated that premature phase separation will occur during synthesis leading to compositional heterogenity and a low molecular weight. This is borne out by experiment and typically the tensile strength and elongation at break of PDMS-based polyurethanes is about 7 MPa and 200%, respectively6.
Several techniques have been reported in the literature to improve mechanical properties of PDMS-based polyurethanes with the primary focus being increasing the interfacial adhesion between soft PDMS phase and hard segments. These include mixing with certain polyethers or polyesters7, (b) introduction of polar functionality to PDMS (c) use of copolymers of PDMS and polyethers as soft segments and (d) hard segment modifications. Only marginal improvements in mechanical properties have been observed using these techniques.
A requirement accordingly exists to develop methods for incorporating polysiloxane segments as part of the polyurethane structure to yield materials with good mechanical properties. The current demand for materials with improved biocompatibility and stability warrants development of polysiloxane-containing polyurethanes, especially those that are resistant to degradation when implanted for long periods of time.
According to the present invention there is provided a material having improved mechanical properties, clarity, processability and/or degradation resistance comprising a polyurethane elastomeric composition which includes a soft segment derived from a polysiloxane macrodiol and a polyether and/or polycarbonate macrodiol.
It will be appreciated that more than one polysiloxane macrodiol and polyether and/or polycarbonate macrodiol may be present in the polyurethane elastomeric composition.
The present invention also provides use of the polyurethane elastomeric composition defined above as a material having improved mechanical properties, clarity, processability and/or degradation resistance.
The present invention further provides the polyurethane elastomeric composition defined above when used as a material having improved mechanical properties, clarity, processability and/or degradation resistance.
The mechanical properties which are improved include tensile strength, tear strength, abrasion resistance, Durometer hardness, flexural modulus and related measures of flexibility.
The improved resistance to degradation includes resistance to free radical, oxidative, enzymatic and/or hydrolytic processes and to degradation when implanted as a biomaterial.
The improved processability includes ease of processing by casting such as solvent casting and by thermal means such as extrusion and injection molding, for example, low tackiness after extrusion and relative freedom from gels.
There is also provided a degradation resistant material which comprises the polyurethane elastomeric composition defined above.
The polyurethane elastomeric composition of the present invention shows significantly improved degradation resistance over the softer grades disclosed in U.S. Pat. No. 4,875,308 and the commercially available polyurethane Pellethane 2363-80A (Registered Trade Mark). It also has a good compatibility and stability in biological environments, particularly when implanted in vivo for extended periods of time.
According to another aspect of the present invention there is provided an in vivo degradation resistant material which comprises the polyurethane elastomeric composition defined above.
The polyurethane elastomeric composition may also be used as a biomaterial. The term xe2x80x9cbiomaterialxe2x80x9d is used herein in its broadest sense and refers to a material which is used in situations where it comes into contact with the cells and/or bodily fluids of living animals or humans.
The polyurethane elastomeric composition is therefore useful in manufacturing medical devices, articles or implants.
Thus, the present invention still further provides medical devices, articles or implants which are composed wholly or partly of the polyurethane elastomeric composition defined above.
It will be understood that the polyurethane elastomeric composition may be used as a coating on medical devices, articles or implants.
The medical devices, articles or implants may include cardiac pacemakers and defibrillators, catheters, cannulas, implantable prostheses, cardiac assist devices, heart valves, vascular grafts, extra-corporeal devices, artificial organs, pacemaker leads, defibrillator leads, blood pumps, balloon pumps, A-V shunts, biosensors, membranes for cell encapsulation, drug delivery devices, wound dressings, artificial joints, orthopaedic implants and soft tissue replacements.
It will be appreciated that polyurethane elastomeric compositions having properties optimised for use in the construction of various medical devices, articles or implants will also have other non-medical applications. Such applications may include their use in the manufacture of artificial leather, shoe soles; cable sheathing; varnishes and coatings; structural components for pumps, vehicles, etc.; mining ore screens and conveyor belts; laminating compounds, for example in glazing; textiles; separation membranes; sealants or as components of adhesives.
Thus, the present invention extends to the use of the polyurethane elastomeric composition defined above in the manufacture of devices or articles.
The present invention also provides devices or articles which are composed wholly or partly of the polyurethane elastomeric composition defined above.
Some of the polyurethane elastomeric compositions of the present invention are novel per se. These novel compositions exhibit unexpectedly improved clarity, processability, flexural modulus, mechanical properties, abrasion resistance, softness and/or resistance to degradation which make them suitable for a wide range of applications.
Thus, the present invention further provides a polyurethane elastomeric composition which includes macrodiols derived from 60 to 98 wt %, more preferably 70 to 90 wt % of a polysiloxane macrodiol and 2 to 40 wt %, more preferably 10 to 30 wt % of a polyether and/or polycarbonate macrodiol.
Suitable polysiloxane macrodiols are hydroxy terminated and include those represented by the formula (I) 
wherein
R1, R2, R3, R4, R5 and R6 are same or different and selected from an optionally substituted straight chain, branched or cyclic, saturated or unsaturated hydrocarbon radical; and
n is an integer of 1 to 100.
A preferred polysiloxane is PDMS which is a compound of formula (I) wherein R1 to R4 are methyl and R5 and R6 are as defined above. Preferably R5 and R6 are the same or different and selected from propylene, butylene, pentylene, hexylene, ethoxypropyl (xe2x80x94CH2CH2OCH2CH2CH2xe2x80x94), propoxypropyl and butoxypropyl.
The hydrocarbon radical for substituents R1, R2, R3 and R4, may include alkyl, alkenyl, alkynyl, aryl or heterocyclyl radicals. It will be appreciated that the equivalent radicals may be used for substituents R5 and R6 except that the reference to alkyl, alkenyl and alkynyl should be to alkylene, alkenylene and alkynylene, respectively. In order to avoid repetition, only detailed definitions of alkyl, alkenyl and alkynyl are provided hereinafter.
The term xe2x80x9calkylxe2x80x9d denotes straight chain, branched or mono- or poly-cyclic alkyl, preferably C1-12 alkyl or cycloalkyl. Examples of straight chain and branched alkyl include methyl, ethyl, propyl, isopropyl, butyl, isobutyl, sec-butyl, tert-butyl, amyl, isoamyl, secamyl, 1,2-dimethylpropyl, 1,1-dimethylpropyl, pentyl, hexyl, 4-methylpentyl, 1-methylpentyl, 2-methylpentyl, 3-methylpentyl, 1,1-dimethylbutyl, 2,2-dimethylbutyl, 3,3-dimethylbutyl, 1,2-dimethylbutyl, 1,3-dimethylbutyl, 1,2,2-trimethylpropyl, 1,1,2-trimethylpropyl, heptyl, 5-methylhexyl, 1-methylhexyl, 2,2-dimethylpentyl, 3,3-dimethylpentyl, 4,4-dimethylpentyl, 1,2-dimethylpentyl, 1,3-dimethylpentyl, 1,4-dimethylpentyl, 1,2,3-trimethylbutyl, 1,1,2-trimethylbutyl, 1,1,3-trimethylbutyl, octyl, 6-methylheptyl, 1-methylheptyl, 1,1,3,3-tetramethylbutyl, nonyl, 1-, 2-, 3-, 4-, 5-, 6- or 7-methyloctyl, 1-, 2-, 3-, 4- or 5-ethylheptyl, 1-, 2- or 3-prophylhexyl, decyl, 1-, 2-, 3-, 4-, 5-, 6-, 7- and 8-methylnonyl, 1-, 2-, 3-, 4-, 5- or 6-ethyloctyl, 1-, 2-, 3- or 4-propylheptyl, undecyl 1-, 2-, 3-, 4-, 5-, 6,7-, 8- or 9-methyldecyl, 1-, 2-, 3-, 4-, 5-, 6- or 7-ethylnonyl, 1-, 2-, 3-, 4- or 5-propyloctyl, 1-, 2- or 3-butylheptyl, 1-pentylhexyl, dodecyl, 1-, 2-, 3-, 4-, 5-, 6-, 7-, 8-, 9- or 10-methylundecyl, 1-, 2-, 3-, 4-, 5-, 6-, 7- or 8-ethyldecyl, 1-, 2-, 3-, 4-, 5- or 6-propylnonyl, 1-, 2-, 3- or 4-butyloctyl, 1,2-pentylheptyl and the like. Examples of cyclic alkyl include cyclopropyl, cyclobutyl, cyclopentyl, cyclohexyl, cycloheptyl, cyclooctyl, cyclononyl and cyclodecyl and the like.
The term xe2x80x9calkenylxe2x80x9d denotes groups formed from straight chain, branched or mono- or poly-cyclic alkenes including ethylenically mono- or poly-unsaturated alkyl or cycloalkyl groups as defined above, preferably C2-12 alkenyl. Examples of alkenyl include vinyl, allyl, 1-methylvinyl, butenyl, iso-butenyl, 3-methyl-2-butenyl, 1-pentenyl, cyclopentenyl, 1-methyl-cyclopentenyl, 1-hexenyl, 3-hexenyl, cyclohexenyl, 1-heptenyl, 3-heptenyl, 1-octenyl, cyclooctenyl, 1-nonenyl, 2-nonenyl, 3-nonenyl, 1-decenyl, 3-decenyl, 1,3-butadienyl, 1-4, pentadienyl, 1,3-cyclopentadienyl, 1,3-hexadienyl, 1,4-hexadienyl, 1,3-cyclohexadienyl, 1,4-cyclohexadienyl, 1,3-cycloheptadienyl, 1,3,5-cycloheptatrienyl, 1,3,5,7-cycloocta-tetraenyl and the like.
The term xe2x80x9calkynylxe2x80x9d denotes groups formed from straight chain, branched, or monoor poly-cyclic alkynes. Examples of alkynyl include ethynyl, 1-propynyl, 1- and 2-butynyl, 2-methyl-2-propynyl, 2-pentynyl, 3-pentynyl, 4-pentynyl, 2-hexynyl, 3-hexynyl, 4-hexynyl, 5-hexynyl, 10-undecynyl, 4-ethyl-1-octyn-3-yl, 7-dodecynyl, 9-dodecynyl, 10-dodecynyl, 3-methyl-1-dodecyn-3-yl, 2-tridecynyl, 1 1-tridecynyl, 3-tetradecynyl, 7-hexadecynyl, 3-octadecynyl and the like.
The term xe2x80x9carylxe2x80x9d denotes single, polynuclear, conjugated and fused residues of aromatic hydrocarbons. Examples of aryl include phenyl, biphenyl, terphenyl, quaterphenyl, phenoxyphenyl, naphthyl, tetrahydronaphthyl, anthracenyl, dihydroanthracenyl, benzanthracenyl, dibenzanthracenyl, phenanthrenyl and the like.
The term xe2x80x9cheterocyclylxe2x80x9d denotes mono- or poly-cyclic heterocyclyl groups containing at least one heteroatom atom selected from nitrogen, sulphur and oxygen. Suitable heterocyclyl groups include N-containing heterocyclic groups, such as, unsaturated 3 to 6 membered heteromonocyclic groups containing 1 to 4 nitrogen atoms, for example, pyrrolyl, pyrrolinyl, imidazolyl, pyrazolyl, pyridyl, pyrimidinyl, pyrazinyl, pyridazinyl, triazolyl or tetrazolyl; saturated 3 to 6-membered heteromonocyclic groups containing 1 to 4 nitrogen atoms, such as pyrrolidinyl, imidazolidinyl, piperidino or piperazinyl; unsaturated condensed heterocyclic groups containing 1 to 5 nitrogen atoms, such as, indolyl, isoindolyl, indolizinyl, benzimidazolyl, quinolyl, isoquinolyl, indazolyl, benzotriazolyl or tetrazolopyridazinyl; unsaturated 3 to 6-membered heteromonocyclic group containing an oxygen atom, such as, pyranyl or furyl; unsaturated 3 to 6-membered heteromonocyclic group containing 1 to 2 sulphur atoms, such as, thienyl; unsaturated 3 to 6-membered heteromonocydic group containing 1 to 2 oxygen atoms and 1 to 3 nitrogen atoms, such as, oxazolyl, isoxazolyl or oxadiazolyl; saturated 3 to 6-membered heteromonocyclic group containing 1 to 2 oxygen atoms and 1 to 3 nitrogen atoms, such as, morpholinyl; unsaturated condensed heterocyclic group containing 1 to 2 oxygen atoms and 1 to 3 nitrogen atoms, such as, benzoxazolyl or benzoxadiazolyl; unsaturated 3 to 6-membered heteromonocyclic group containing 1 to 2 sulphur atoms and 1 to 3 nitrogen atoms, such as, thiazolyl or thiadiazolyl; saturated 3 to 6-membered heteromonocyclic group containing 1 to 2 sulphur atoms and 1 to 3 nitrogen atoms, such as, thiadiazolyl; and unsaturated condensed heterocyclic group containing 1 to 2 sulphur atoms and 1 to 3 nitrogen atoms, such as benzothiazolyl or benzothiadiazolyl.
In this specification, xe2x80x9coptionally substitutedxe2x80x9d means that a group may or may not be further substituted with one or more groups selected from oxygen, nitrogen, sulphur, alkyl, alkenyl, alkynyl, aryl, halo, haloalkyl, haloalkenyl, haloalkynyl, haloaryl, hydroxy, alkoxy, alkenyloxy, alkynyloxy, aryloxy, carboxy, benzyloxy, haloalkoxy, haloalkenyloxy, haloalkynyloxy, haloaryloxy, nitro, nitroalkyl, nitroalkenyl, nitroalkynyl, nitroaryl, nitroheterocyclyl, azido, amino, alkylamino, alkenylamino, alkynylamino, arylamino, benzylamino, acyl, alkenylacyl, alkynylacyl, arylacyl, acylamino, acyloxy, aldehydo, alkylsulphonyl, aryisulphonyl, alkylsulphonylamino, arylsulphonylamino, alkylsulphonyloxy, arylsulphonyloxy, heterocyclyl, heterocycloxy, helerocyclylamino, haloheterocyclyl, alkylsulphenyl, arylsulphenyl, carboalkoxy, carboaryloxy, mercapto, alkylthio, arylthio, acylthio and the like.
The polysiloxane macrodiols may be obtained as commercially available products such as X-22-160AS from Shin Etsu or prepared according to known procedures8. The preferred molecular weight range of the polysiloxane macrodiol is about 200 to about 6000, more preferably about 400 to about 1500. Polyurethane elastomeric compositions having polysiloxane macrodiols falling within this preferred molecular weight range offer particularly improved clarity and mechanical properties.
Suitable polyether macrodiols include those represented by the formula (II)
HOxe2x80x94[(CH2)mxe2x80x94O]nxe2x80x94Hxe2x80x83xe2x80x83(II) 
wherein
m is an integer of 4 or more, preferably 5 to 18; and
n is an integer of 2 to 50.
Polyether macrodiols of formula (II) wherein m is 5 or higher such as polyhexamethylene oxide (PHMO), polyheptamethylene diol, polyoctamethylene oxide (POMO) and polydecamethylene oxide (PDMO) are preferred over the conventional PTMO. These polyethers, due to their hydrophobic nature, are more miscible with PDMS macrodiols and yield polyurethanes that are compositionally homogeneous, have high molecular weights and display improved clarity.
In a particularly preferred embodiment, the polyurethane elastomeric composition includes a soft segment derived from a polysiloxane macrodiol and a polyether macrodiol of formula (II) defined above.
The polyether macrodiols may be prepared by the procedure described by Gunatillake et al9. Polyethers such as PHMO described in this reference are more hydrophobic than PTMO and are more compatible with polysiloxane macrodiols. The preferred molecular weight range of the polyether macrodiol is about 200 to about 5000, more preferably about 200 to about 1200.
Suitable polycarbonate macrodiols include poly(alkylene carbonates) such as poly(hexamethylene carbonate) and poly(decamethylene carbonate); polycarbonates prepared by reacting alkylene carbonate with alkanediols for example 1,4-butanediol, 1,10-decandiol (DD), 1,6-hexanediol (HD) and/or 2,2-diethyl 1,3-propanediol (DEPD); and silicon based polycarbonates prepared by reacting alkylene carbonate with 1,3-bis (4-hydroxybutyl)-1,1,3,3-tetramethyldisiloxane (BHTD) and/or alkanediols.
It will be appreciated when both the polyether and polycarbonate macrodiols are present, they may be in the form of a mixture or a copolymer. An example of a suitable copolymer is a copoly(ether carbonate) macrodiol represented by the formula (III) 
wherein
R1 and R2 are same or different and selected from an optionally substituted straight chain, branched or cyclic, saturated or unsaturated hydrocarbon radical; and
m and n are integers of 1 to 20.
Although the compound of formula (III) above indicates blocks of carbonate and ether groups, it will be understood that they also could be distributed randomly in the main structure.
The polyurethane elastomeric compositions of the present invention may be prepared by any suitable known technique. A preferred method involves mixing the polysiloxane macrodiol, polyether and/or polycarbonate macrodiol and chain extender and then reacting this mixture with a diisocyanate. The initial ingredients are preferably mixed at a temperature in the range of about 45 to about 100xc2x0 C., more preferably about 60 to about 80xc2x0 C. If desired, a catalyst such as dibutyl tin dilaurate at a level of about 0.001 to about 0.5 wt % based on the total ingredients may be added to the initial mixture. The mixing may occur in conventional apparatus or within the confines of a reactive extruder or continuous reactive injection molding machine.
Alternatively, the polyurethanes may be prepared by the prepolymer method which involves reacting a diisocyanate with the polysiloxane and polyether and/or polycarbonate macrodiols to form a prepolymer having terminally reactive diisocyanate groups. The prepolymer is then reacted with a chain extender.
Thus, the polyurethane elastomeric composition of the present invention may be further defined as including a reaction product of:
(i) macrodiols including:
(a) polysiloxane macrodiol; and
(b) a polyether and/or polycarbonate macrodiol;
(ii) a diisocyanate; and
(iii) a chain extender.
Preferably, the diisocyanate is selected from 4,4xe2x80x2-methylenediphenyl diisocyanate (MDI), methylene bis (cyclohexyl) diisocyanate (H12MDI), p-phenylene diisocyanate (p-PDI), trans-cyclohexane-1,4-diisocyanate (CHDI) or a mixture of the cis and trans isomers, 1,6-hexamethylene diisocyanate (DICH), 2,4-toluene diisocyanate (2,4-TDI) or its isomers or mixtures thereof, p-tetramethylxylene diisocyanate (p-TMXDI) and m-tetramethylxylene diisocyanate (m-TMXDI). MDI is particularly preferred.
The chain extender is preferably selected from 1,4-butanediol, 1,6-hexanediol, 1,8-octanediol, 1,9-nonadiol, 1,10-decanediol 1,4-cyclohexane dimethanol, p-xyleneglycol, 1,4-bis (2-hydroxyethoxy) benzene and 1,12-dodecanediol. 1,4-butanediol is particularly preferred.
The methods described above here do not generally cause premature phase separation and yield polymers that are compositionally homogeneous and transparent having high molecular weights. These methods also have the advantage of not requiring the use any solvent to ensure that the soft and hard segments are compatible during synthesis.
A further advantage of the incorporation of the polysiloxane segment according to the present invention is the relative ease of processing of the polyurethane by conventional methods such as extrusion, injection and compression moulding without the need of added processing waxes. If desired, however, conventional polyurethane processing additives such as catalysts for example dibutyl tin dilaurate (DBTD), stannousoxide (SO), 1,8-diazabicyclo[5,4,0] undec-7-ene (DABU), 1,3-diacetoxy-1,1,3,3,-tetrabutyldistannoxane (DTDS), 1,4-diaza-(2,2,2)-bicyclooctane (DABCO), N,N,Nxe2x80x2,Nxe2x80x2-tetramethylbutanediamine (TMBD) and dimethyllin dilaurate (DMTD); antioxidants for example Inganox (Registered Trade Mark); radical inhibitors for example trisnonylphenyl phosphite (TNPP); stabilisers; lubricants for example Irgawax (Registered Trade Mark); dyes; pigments; inorganic and/or organic fillers; and reinforcing materials can be incorporated into the polyurethane during preparation. Such additives are preferably added to the macrodiol mixture.
The polysiloxane macrodiol, polyether macrodiol, diisocyanate and the chain extender may be present in certain preferred proportions. The preferred level of diisocyanate and chain extender in the composition is about 30 to about 60 wt %, more preferably about 30 to about 50 wt %, most preferably about 40 wt %. The weight ratio of polysiloxane to polyether and/or polycarbonate may be in the range of from 1:99 to 99:1. A particularly preferred ratio of polysiloxane to polyether and/or polycarbonate which provides increased degradation resistance and stability is 80:20.
A particularly preferred polyurethane elastomeric composition includes a soft segment derived from 80 wt % of PDMS and 20 wt % of PHMO or PTMO and a hard segment derived from MDI and BDO.
The invention will now be described with reference to the following examples. These examples are not to be construed as limiting the invention in any way.